Stretching is integral to getting the most out of your exercise programme - as swimming is an all-body workout, try to stretch all of the major muscle groups before you swim.
The best way to do this is to stretch each body part in order - holding stretches for 10 to 15 seconds - and run through this routine three times.
Stretching cold muscles may relieve tension but will have very little effect on flexibility so gently swim for five minutes before undertaking your stretches.
If you’re stretching in the water, your body will cool down rapidly so maintain your temperature by walking, jogging on the spot or swinging your arms or legs for 20 seconds between each stretch.
You should also stretch after training – try doing this in a warm shower, holding each stretch for 30 - 40 seconds to help clear waste products from the muscles, improve post-exercise flexibility and stimulate the muscle receptors that promote relaxation.
Extracted from the ASA Website Warm up and Stretch